Linux - Signals and Traps

Signals List

There is a simple method for compiling a list of all the signals which your system supports. You can use the command given below to see all the allowed signals:

kill -l 

The following table lists out standard signals you might encounter in Linux signals and traps.

Default Actions

There is a default response for each signal in Linux signals and traps. The action that a script or program executes in response to a signal is known as the default action for a signal.

Several of the potential default behaviors include the following -

• Stopping the procedure.
• Disregard the signal.
• Remove the core: By doing this, a core file contains the process's memory image when the signal is received.
• Stop the process.
• Carry on a halted procedure.

Sending Signal

One of the most popular ways for a user to interrupt a script by pressing Cmd+C/Ctrl+C.

When you press the Cmd+C/Ctrl+C key, a SIGINT will be given to the script, and the script terminates as per the default acion.

The kill command is another typical mechanism for sending signals.

$ signal pid kill

Trapping Signal

Signals can be easily trapped. The shell program often ends immediately, and the command prompt reappears when we press Ctrl + C or the broken key on the terminal while it is running. It might only sometimes be a good idea. We may, for instance, continue to use a set of temporary files that won't be deleted. 

A trap is primarily used in one of two ways in shell scripts in Linux signals and traps:

• Get rid of temporary files.
• Reject the signal.

Null Signal

Investigating the process's existence is a fascinating use case for sending signals in Linux signals and traps. No signal is delivered when the kill() method is invoked with the signal parameter set to 0, which is a null signal. Instead, error checking is used to determine if the process may signal the process. This implies that we can use this approach to detect the existence of the process. Any of the following outcomes can happen when a null signal is sent:

1. If the ESRCH error occurs, the target process does not shut down.
2. The target process must be present and accessible for the caller to deliver a signal to the target if the call is successful.
3. The target process is present if the EPERM error occurs, but we need more authorization to transmit the signal to the process.

Ignoring Signals

The command trap, followed by two empty quotes, can be used as a command to tell the shell to ignore specific signals. 

Any of the following commands will display the shell process that will disregard those signals:

$ trap " " 2 3
$ trap "" INT QUIT

Each signal can be denoted using either signal short or their signal id so that they can be ignored by the shell. 

Trap

When the shell receives signals or under other unique circumstances, the trap specifies and activates handlers to run in Linux signals and traps. The shell will execute the specified and activated handlers when the trap specifies or activates signals or under other unusual conditions.

The shell must read and execute the command ARG when it gets the signal(s) SIGNAL SPEC. Each signal specified is reset to its initial value if ARG is empty (and only one SIGNAL SPEC is provided) or ARG is a dash ("-"). The shell ignores each SIGNAL SPEC and the instructions it executes if ARG is the null string.

Incase the SIGNAL SPEC is EXIT (0), ARG is run when the shell exits. ARG is run before every basic command if a SIGNAL SPEC is DEBUG. Whenever a shell function or a script runs using the built-in commands "." or the source completes running, ARG is performed if a SIGNAL SPEC is a RETURN. When the -e option is enabled, a SIGNAL SPEC of ERR implies that every time a command fails and the shell would otherwise exit, ARG will be executed.

Example

Trap trap 1 7 2 3
cleanup()
{
  echo "trap alert"
  rm -rf /tmp/temp_*.$$
  echo "completed"
  exit 1
}
for i in all
do
  sed s/FOO/BAR/g $i > /tmp/temp_${i}.$$ && mv /tmp/temp_${i}.$$ $i
done

Resetting traps

If you use the trap and remove the first parameter after changing the default action to be executed in response to a signal, you can modify it back. 

For example

$ trap 4 8

By doing this, the default behavior for what should happen when signals 4 or 8 are received is restored.

Frequently Asked Questions

Linux has how many signals?

There are 31standard signalsin total, which are numbered from Numbered 1 - 31. Every signal has the letters "SIG" and a suffix indicating the type of signal . The Linux kernel has 33 distinct real-time signals supported as of version 2.2.

What are signal processing's two primary purposes?

The two main functions of the signal processing hardware are compressing sensed data and identifying significant patterns in recorded data.

What is the trap's purpose in bash?

If the script terminates due to an interruption, the trap command is typically used to clear away temporary files.

Which signal captures the trap?

Use the trap command to capture the SIGINT signal is a typical case.

What signals are impossible to catch?

SIGKILL and SIGSTOP are two signals that cannot be catched.

Conclusion 

In this blog, we have discussed Linux signals and traps with a brief description of their various types. Further we also discussed the process of sending, trapping and resetting signals.

If you think this blog has helped you enhance your knowledge of the above question, and if you want to learn more, check out our articles. Visit our website to read more such blogs. 

1. Introduction to Linux
2. Linux Kernel
3. Linux Operating System

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